Pyrolytic plugging of oil-bearing formations by underground combustion



United States Patent Inventor Lloyd E. Elkins Tulsa, Okla.

Appl. No. 834,900

Filed June 19, 1969 Patented Dec. 15, 1970 Assignee Pan AmericanPetroleum Corporation Tulsa, Okla. a corporation of Delaware PYROLYTICPLUGGING OF OIL-BEARlNG FORMATIONS BY UNDERGROUND COMBUSTION 8 Claims,No Drawings U.S. Cl 166/261,

Int. Cl E2lb 43/24 Field of Search 166/288,

Primary ExaminerStephen .l. Novosad Attorneys-Paul F. l-lawley and BeullB. Hamilton ABSTRACT: In an underground forward combustion process forrecovering petroleum from earth formations, more uniform progress of thecombustion front through a formation over a wide area is obtained byinjecting a water solution of sugar after the combustion operation hasbeen started. The sugar undergoes pyrolysis forming a cohesive solidmass when it becomes heated near the combustion front. The oxidizing gastends to char this solid, further increasing the plugging of channelsand thus providing a more uniform progress of the combustion front.Other organic materials can be used in place of sugar if they undergopyrolysis to deposit cohesive solids.

PYROLYTIC PLUGGING OF OIL-BEARING FORMATIONS BY- UNDERGROUND COMBUSTIONMany methods are used to recover oil from oil-bearing undergroundformations. in one method, air is injected into the formation and theoil is ignited at the injection well. Continued injection of air causesa combustion front to move through the formation forcing oil ahead ofthe front to produc ing wells through which the oil is produced to thesurface. The combustion front naturally moves most rapidlythrough themost permeable zones of the formation. The front reaches the producingwells through the more permeable zones before all the oil is recoveredfrom the lesspermeable zones. Even in any one zone of uniformpermeability, the front tends to streak through in an approximatelystraight line from an injection well to aproducing well without fanningout very far laterally in the zone. In addition, the movement of thecombustion front is inherently unstable even in zones of uniformpermeability. Once a small section of the front gets ahead of theremainder of the front, this small sectiontends to move ahead morerapidly than the remainder of the frontThis is because the permeabilityof the formation behind the frontis veryhigh compared to thepermeability ahead of the front. When a small section of the combustionfront gets ahead of the remainder of the front, there is a shorterlength of the low permeability portion ahead of this section thanaheadof the remaining front. Therefore, flow of fluids is more rapid alongthe path including this shorter length of low permeability ahead of theadvanced section of combustion front and this section moves ahead evenmore rapidly.

Before a combustion operation is started, movement of the front in zonesof different permeability can be made more uniform by injecting amaterial to form a partial plug in the formation. More of the pluggingmaterial goes into the highly permeable zonesthan into the permeablezones, thus plugging the more permeable zones to a greater degree. Theplugging materials for this purpose may be foams,'activ'ated silicates,finely divided solids, settable liquid resins, or the like.

Even though a partial plugging operation isused before combustion isinitiated, nonuniform progress of the combustion frontmay develop evenin a zone of uniform permeability, as explained above. The problem is todecrease the nonuniform progress of the font after the combustionoperation has started. if ordinary plugging techniques are used in theburned-out region around the injection well, the high temperaturecomplicates control of the plugging operations. if a plug is formed nearthe injection well, the-injected gases immediately rearrange themselvesinto previous patterns as soon as they have penetrated the pluggedvolume and enter the highly permeable burned-out formation between theplug and the combustion front. If a plug is formed near the combustionzone, control problems due to higher temperatures become extreme.

An object of this invention is to provide a method for forming a partialplug in the burned-out portion of a formation behind an undergroundforward combustion front. A more specific object is to provide a processfor forming a partial plug in the burned-out portion of a formation nearan underground forward combustion front. Still other objects will beapparent to the those skilled in the art in view of the followingdescription and claims.

in general, I accomplish the objects of my invention by injecting theinjection well and into the formation a water solution of a material,such as a sugar, which undergoes pyrolysis at high temperatures to forma solid cohesive mass in the pores. of the formation. Thus, advantage istaken of the high tem-- perature to form the plug near the combustionzone where it is most helpful. This process takes advantage of the hightem perature which is a disadvantage in other processes.

Most organic materials undergo pyrolysis at high temperatures. The usualresult is a decomposition of the molecules into small fragmen s whichburn in the oxidizing atmosphere of the underground combustionoperation. in some organic compounds, such as the sugars, however, thefirst pyrolysis reaction is a polymerization. in the case of sucrose,for example, water is eliminated frorntwo hydroxyl groups leading twocarbon atoms connected through an ether linkage. Since sugarmoleculeshave several hydroxyl, groups, extensive polymerization occurs. Otherpblyhydroxylorganic .compounds are also known to undergo similarpolymerization at high temperatures. These include the so-called sugaracids, such as gluconic acid, mannuronic acid, 'galacturonic acid, andthe like. The sugar alcohols, such as sorbitol and mannitol, as well asother polyhydroxy compounds, such as pentaerythritol, and evenglycerine, are members of another group of satisfactory products.

Polyamines, such as triaminobenzene, tetraethylene pentamine, and thelike, can lose ammonia from two amine groups on heating and formpolymers with carbon atoms of different molecules connected throughamine linkages. Polyunsaturated acids, such as linoleic and linolenicacids, are known to polymerize upon heatingfWater-soluble salts of suchacids may also be satisfactory under some conditions. Stillothermaterials which polymerize upon heating will occur to those skilledin the art.

It should be noted that polymerization at high temperature is not theonly requirement of the organic compound. it must be soluble to theextent of at least about 1 percent by weight in water. Also, a watersolution of the material must be capable of being injected intopermeableearth formations. Most natural gums, such as karaya and starch, as wellas many synthetic materials,-suchas sodium carboxymethyl cellulose, formwhat might be considered water solutions, but these are used indrillingfluids, for example, because they will not enter formations butdeposit impermeable cakes on-the face'of the formation. Such materialsare not,. of course, satisfactory in my process.

Some organic materials polymerize at temperatures too low for mypurpose. For example, the low-temperature reaction product of phenol andformaldehyde polymerizes upon heating to form a cohesive, solid pluggingmaterial. Since this reaction takes place at a temperature of only 300"or 400 F., it would deposit in the burned-out formation too far behindthe combustion front to be very effective. The organic material shouldpyrolyze to form'asolid at a temperature between about 400 F. and about800 F. if pyrolysis takes place at a lower temperature, prematureplugging can occur too far away from the combustion zone. If pyrolysisdoes not take place until a temperature of about 800 F. is reached, theplug can form so close to the combustion front that there isdanger it'will be quickly burned out in the highly oxidizing atmosphere.

Summarizing these conditions, the plugging agent must be a water-solubleorganic material capable of entering permeable formations in watersolution and undergoing pyrolysis at a temperature between about 400 F.and about 800 F. to form a cohesive solid mass in the pores of theformation.

The. suitability of a material can be easily determined by passing adilute aqueous solution (2 to 10 percent by weight) of the materialthrough a sand-packed tube while heating the tube to a temperature of400 to 800 F. If the tube becomes plugged, the material is, of course,satisfactory for my purposes.

While any of the above-mentioned classes of materials can be used in myprocess under at least some conditions, the two materials whicharegreatly preferred for .my purposes are sucrose and glucose, preferablyin the form of raw sucrose and corn syrup, respectively. These sugarsare very inexpensive, particularly in the raw form, but are sufficientlypure to permit their solutions to be injected easily into formations.Some natural sugar solutions, such as molasses, are very attractivefromthestandpoint of cost, but most of them must be carefully filteredbefore use to avoid plugging the formation face of the injection well. I

A tube-was'filled with tar sand from the Athabasca field in Alberta,Can. The tar had been previously burned from this sand. A 12-inch lengthof the tube was surrounded with a heater. Thermocouples were attached tothis section of the tube so the temperature could be measured. Asolution of molasses in water was injected into the packed tube togetherwith air. The molasses solution contained about percent sugars byweight. This solution was carefully filtered before use to avoidplugging the tube with solids suspended in the molasses. Flow rates wereabout 3 milliliters per hour for the molasses and about 29 millilitersper hour for the air. The air volume was measured at means pressure inthe tube. A back pressure of about 800 pounds per square inch gauge washeld on the tube. The temperature was increased in steps and thepressure differential across the tube at constant flow volumes wasmeasured. At temperatures up to 400 F., the pressure drop remained atabout 6 p.s.i.g. When the temperature was raised to 600 F., the pressuredrop increased slowly to 85 p.s.i.g. in 72 hours. Raising thetemperature to 700 F. resulted in a sharp increase of pressure drop toabout 400 p.s.i.g. in about 8 hours.

When the flow test was repeated with a 200 p.s.i.g. back pressure, therapid pressure drop occurred at about 530 F. Study of all theinformation indicated that rapid plugging occurred at approximately thetemperature at which the water flashed into steam. Apparently, littlepyrolytic dehydrolysis of sugars can take place while the sugars aredissolved in liquid water. As soon as the water flashes into the vaporphase, however, pyrolytic loss of water from the sugar occurs withconsequent polymerization and formation of a cohesive solid mass whichplugs the formation pores.

A repeat of the second test using nitrogen in place of air resulted inplugging at about the same temperature but somewhat more slowly. Thisshows that the charring action of the oxygen in the air is helpful inthe plugging operation.

A repeat of the second test, using only enough molasses to provide 4percent sugars in the solution, produced the same results as in thesecond test, except that the lower sugar concentration formed the plugmore slowly. This test was terminated and the tube was cooled beforecomplete plugging could take place. Flow of air at various temperatureswas then measured to-determine the stability of the plug. Results areshown in Table I.

TABLE I Effective permeability Temperature, F.: to air, md. 76 06 300 22400 300. 00 600-625 1 250. 00 700 1, 440. 00 800 4, 050. 00

The final permeability at 800 F. was only slightly less than theoriginal permeability before plugging. It will be obvious, therefore,that'if the plug does not form at a temperature below about 800 F.,there is some danger that the plug will be burned out before it can domuch good. It should be noted, however, that about 5,000 pore volumes ofair passed through the plugged zone in this test. so it is unlikely thata well-formed plug would burn out prematurely at 800 F. in actualoperations. As previously explained, however, materials which pyrolyzebelow about 800 F. should be used to avoid this danger.

In another test, no gas was injected with the molasses solution. In thistest, a very dilute solution of molasses was used of sugar, to the lackof air, or to the formation of the plug at very low temperature. Sincein an actual application of the plugging technique, the temperature willnot increase after the plug formed, the use of a batch of molassessolution followed by air is obviously a satisfactory way of forming aneffective plug in the formation behind the combustion front.

In still another test using no gas with the injected molasses solution,a plug was formed very slowly at 600 F. using a molasses solution sodilute that it contained only a little over 1 percent sugars. This plugwas also somewhat unstable when subjected to compressed air at 600 F. Itisobvious from this test that the sugar concentration in the pluggingsolution should be at least about 1 percent by weight and preferablyconsiderably higher.

Concentrated solutions of organic materials, such as sugars, are veryviscous. lf sugar solutions are to be injected at a high rate into andthrough earth formations, the sugar concentration should be not morethan about 20 percent be weight. A concentration as low as 1 percent hasbeen shown to give at least some plugging. Preferably, the sugarconcentration should be about 2 to 10 percent by weight. The sugar maybe a single sugar or a mixture of sugars. Mixtures of other pyrolyticplugging materials with each other or with sugars can also be used.

When a treatment is being designed, the volume of sugar solution willdepend upon the concentration of sugar in the solution and the volume ofpore space to be plugged. Since the object is to plug streaks, a muchsmaller volume of solution can be used than in some processes where theentire formation is to be plugged. if, for example, a streak 12 feethigh and 20 feet wide is to be plugged for a distance of 1 foot, and ifthe porosity is 36 percent, the volume of pore space is about 86.5 cubicfeet. This volume of sucrose with a density of 1.58 grams per milliliterweighs about 8500 pounds. If this amount of sugar is injected as a tenpercent by weight solution, the solution weight is about 85,000 poundsand the volume is about 10,000 gallons, or approximately 240 barrels (42US. gallons per barrel). If a 2 percent solution is to be used, thevolume which provided only about 2 percent by weight of sugars. ln

should be five times as great. If a 20 percent solution is to be used,the volume should be one-half as great. If an injection well issurrounded by four producing wells and four streaks are expected, thevolume should be about four times as great. The volume should also beadjusted for the porosity of the formation to be treated for theestimated width and height of the streak and for the length of thedesired plug.

As the sugar begins to undergo pyrolysis and deposite in the pores ofthe main flow channels, flow through these pores is restricted and isincreasingly diverted to other flow paths. Therefore, a complete plug isnot necessary. A partial plug provides considerable benefits. If acomplete plug of any length forms, even a fraction of an inch, flow iscompletely diverted from the plugged flow channel and the plug lengthdoes not increase. it will be apparent that a complete plug a foot longwould be difficult to form in my process. For these reasons, the volumesof sugar solution suggested above are larger than necessary to produceat least some benefits.

Also, as the sugar undergoes pyrolysis, it foams. Of course, a smallervolume of solid in the form of foam is required to fill a given porevolume than if the solid is in a dense unfoamed state. Again, this meansthat the volumes of solution suggested above are somewhat larger thanare actually required.

The solution may be injected as a single batch, or it may be injectedcontinuously with the injected air. The treatment may be repeated toplug new channels as they form during the combustion operation.

A considerable volume of water should follow the sugar solution into theformation. If the solution is simply injected into the formation and isthen followed by air only, the solution will remain near the well boreto establish the irreducible minimum water saturation of from about 10to about 30 pershould be injected after the sugar solution to displacethe solution ahead toward the combustion zone. The displacing water maybe alternated with air.

In one very advantageous way of conducting an underground combustionoperation, air and water are alternately injected into the formation. Insuch a process, a batch of sugar solution can be occasionallysubstituted for one of the batches of water to obtain the pluggingaction of the sugar near the combustion zone.

it is possible to use the process to form a plug by injecting thesolution into the formation from the producing well. A heated zoneprecedes a forward combustion front for some distance in the formation.When the'sugar solution reaches a hot enough portion of this heatedzone, a plug will be formed. It should be noted, however, that the plugwill be formed near the combustion zone which will quickly move throughthe plugged zone. At this time, the plug will be burned out. Therefore,it is usually best to use other plugging techniques to reduce thepermeability of high-flow capacity channels at the producing well. itwill be apparent that my process can be used in combination with otherplugging processes applied to either a producing well or an injectionwell. For example, foams can be used to provide a more uniforminjectivity of air into zones of different permeability, these foamsbeing used before the combustion operation begins. My process can thenbe used to plug channels which form after the combustion operation hasstarted.

My process obviously is applicable only to forward combustion operationswhere the combustion zone moves in the direction of flow of injectedfluids. it is not applicable to reverse combustion operation where thesugar solution would not pass through a portion of the formation hotenough to form a plug before the solution reached the combustion zonewhere the sugar would burn.

My process will be better understood from the following example. Aninjection well is surrounded by four producing wells. All the wellspenetrate an oil-bearing formation about 100 feet thick. Combustion hasbeen initiated in the oil-bear ing formation at the bottom of theinjection well and air has been injected into this well for about 60days, together with occasional batches of water. in order to reducechanneling of this air through the formation, a water solution ofmolasses is injected into the formation through the injection well. Thesolution contains about 75 percent water and about 25 percent molasses,both percentages being by volume. The molasses contains about 40 percentby weight of sugars, so the final water solution contains about percentby weight of sugars. The water solution is carefully filtered before useto avoid plugging the face of the oil-bearing formation at the injectionwell. The volume of solution is about 20,000 gallons. As soon as thesolution is introduced into the injection well, injection of air withoccasional batches of water is resumed. The molasses treatment isrepeated every 60 days until breakthrough of combustion zone occurs intothe producing wells. This takes place about 14 months after initiatingthe combustion. The time of breakthrough and the oil productionrecovered to breakthrough are greatly increased over the values whichare usual without the plugging action.

Several alternates and variations have been described. These arepresented by way of example only. Still other alternates and variationswill occur to those skilled in the art. Therefore, I do not wish to belimited to the examples given but only by the following claims.

lclaim:

1. In a forward combustion process in which an oxygen-containing gas isinjected through an injection well and into an oilbearing formation tosupport a combustion zone which moves through the formation displacingoil ahead of the combustion zone to a producing well from which oil'isproduced to the surface, the improvement comprising injecting throughsaid injection well and into said formation after the combustionsatisfactory proceeded for some time, and while combustion iscontinuing, an aqueous solution of anorganic material capable ofentering the pores of the formation, said material being pyrolyzable ata temperature between about 400F. and about 800F. to form a cohesivesolid mass, whereby as said solution approaches the combustion zone, theorganic material pyrolyzes to plug the principal flow channels, thusdiverting flow into other portions of the formation and decreasingchanneling tendencies.

2. The method of claim 1 in which said organic material is filteredmolasses.

3. The method of claim 1 in which said organic material is a polyhydroxycompound.

4. The method of claim 3 in which said polyhydroxy compound is a sugar.

5. The method of claim 4 in which said sugar is sucrose.

6. The method of claim 4 in which the concentration of said sugar isbetween about l and about 20 percent by weight in said aqueous solution.

7. The method of claim 1 in which said solution of an organic materialis followed into said formation by water injected to displace saidsolution ahead toward said combustion zone.

8. The method of claim 7 in which said water is injected as smallbatches alternated with oxygen-containing gas.

